1. Field of the Invention
The present invention relates to a method of evaluating performance of air-fuel ratio sensors and more particularly, to a method of measuring properties of oxygen sensors consisting of a solid electrolyte used for detecting air-fuel ratio of exhaust gas from internal combustion engines and an apparatus for effecting the evaluating method.
2. Related Art Statement
It is desirable that the evaluation of properties of an oxygen sensor which is used in the exhaust gas of an the internal combustion engine in motor vehicles is effected by actually mounting the oxygen sensor to be measured on the internal combustion engine. However, when the measurements are practically effected with the oxygen sensors actually mounted on the internal combustion engines, the results of measurements vary depending on the kind of engine, control systems, sensor set positions, running conditions, and circumferential conditions (temperature, humidity, pressure). Furthermore, there are problems of stability and economics of measurement since a lot of time and steps are required for measuring the properties of the sensors. Therefore, the measurement of properties can not be practically effected on the actual engine.
Hitherto, there have been used two kinds of measuring methods, one of which is a model gas method in which various kinds of gas supplied from bombs is used to prepare a test gas mixture having a composition as similar as possible to the exhaust gas. The sensor and the test gas mixture are electrically heated under a control. Another method is a combustion gas method in which a fuel gas such as propane, city gas or the like is burnt, the burnt gas being used to measure the properties of the oxygen sensor to be measured.
Among the above mentioned measuring methods, the model gas method has excellent on strictness and stability of measuring conditions, but is mainly used in the laboratory field, since it is expensive and the treating capacity is limited. Therefore, the model gas method is not practical for measuring the performance of a number of sensors. On the other hand, the combustion, method using a gas burner is economical and practical and has a high treating capacity for measuring the performance of a number of sensors, but it can not satisfactorily represent the performance measured on the actual engine.
An example of an apparatus adapted for carrying out the conventional combustion gas method is illustrated in FIG. 14, which is known in a thesis "Characteristics of ZrO.sub.2 -Type Oxygen Sensors for Automotive Applications" presented by C. T. Young and J. D. Bode at a society in Detroit, Feb. 26-Mar. 2, 1979 as SAE Technical Paper Series No. 790143.
Referring to FIG. 14, fuel gas and air are supplied from fuel gas supplies 51-1 and 51-2 and air supplies 52-1 and 52-2 via gas flow adjusting solenoid valves 53-1 and 53-2, respectively, to a gas burner 54 to mix the fuel gas with air and then burn the gas mixture. The burnt gas from the gas burner is supplied into a cylindrical port 55 which is provided with an oxygen sensor 56 to be measured. The measured data from the oxygen sensor 56 is processed in a measuring device 57 which outputs signals to control a solenoid valve actuator 58.
According to the above arrangement, burnt gas can be obtained simply by burning the gas mixture in the gas burner 54. Thus, a lot of burnt gas can be supplied in a simple manner. The composition of the burnt gas, however, can only be controlled by changing the air-fuel ratio of fuel gas and air supplied to the gas burner 54.
Therefore, there are disadvantages that the burnt gas can not be perfectly imitated to the exhaust gas from the actual internal combustion engine since the gas mixture is burnt under a complete combustion condition and the amount of unburned gas is slight. Furthermore, in the aforementioned apparatus, only a detecting portion of the oxygen sensor 56 is exposed to a burnt gas flow in the cylindrical port 55 so that the burnt gas flow can not sufficiently contact the detecting portion of the oxygen sensor 56 for a short time and consequently, the properties of the sensor can not be accurately evaluated.